Comparative analysis of wire joining techniques for silver electrodes on piezoceramic bimorphs

Connecting lead wires to the silver electrodes of a piezoceramic disc is a crucial step in manufacturing bimorph deformable mirrors. When manufacturing bimorph deformable mirrors (especially small-aperture), connecting electrical wires is a particular challenge, as the contact pad size is significantly reduced, making it extremely difficult to use traditional wire bonding technology with traditional gluing technology. Conventional soldering methods are also impossible, as heating the piezoelectric disk leads to depolarization of the piezoelectric ceramic, which leads to mirror failure and local initial aberrations arising. 

Three primary welding technologies are typically compared for this application: ultrasonic wire bonding, parallel-gap resistance welding, and laser microwelding.

Ultrasonic wedge bonding uses vibrating sonotrode for a solid-state metallurgical bond between the wire and the silver surface through plastic deformation at temperatures well below the melting point. Its primary advantage is the absence of heat input, which eliminates the risk of locally depolarizing the piezoceramic with Curie temperatures.  Parallel-gap resistance welding passes a high-amperage, low-voltage current through the wire and the electrode simultaneously, when localized Joule heating rapidly melts the interface. Laser micro-welding provides the highest precision using pulsed fiber lasers focused on the wire-electrode junction allows for energy delivery within tens of microseconds. Unlike resistance welding, the beam is highly controllable and can be shaped to minimize lateral heat spread. While safer than resistive techniques regarding bulk heating, peak temperatures still pose a local depolarization risk directly under the joint. Furthermore, the reflective nature of pure silver necessitates either higher power densities or pre-tinning with solders that absorb specific wavelengths better. Thereby, the laser welding technology wasn't applicable for such tasks. In this paper we compared two methods of connection for bimorph deformable mirror such as parallel-gap and ultrasonic welding. The quality of the connection was investigated by tension strength value for both techniques. The tensile strength was compared for both methods. For parallel-gap welding provide value in the range of 0.2-0.25 N, for resistance welding this value was increased up to 0.5 N.